I'm not accessing any web page...this happens as soon as I've turned on the computer & opened my browser. I just click on the popup to close it and that's it until the next time it appears. I keep thinking there's something I should fix, but have no idea what !

Most browsers have a home page which opens as soon as you start the browser. You can change this in the settings of the browser. You might want to try changing this page to another for a while and see if the pop-ups continue.

Is there any chance you may be able to get a screenshot of the popup? I'm wondering if you've possibly got some malware in there affecting your browser. Do you have any antivirus software on your computer?